The present invention relates to a process for the vacuum decarburization of steel by blowing oxygen downwardly upon the molten steel surface under reduced pressure and in particular to such a process using an oxygen lance equipped with special cooling means.
The Witten process of VOD process is a process for refining steel wherein oxygen is blown up the molten steel surface in a closed vessel maintained under reduced pressure while agitating the molten steel by blowing Ar gas through the molten mass to effect decarburization by preventing the oxidation of alloy metals such as Cr, Mn, etc. in the low carbon range. The Witten process is very advantageous economically not only in that the oxidation of Cr, Mn, etc. in the low carbon range is reduced as compared with conventional processes employing, for example, a converter or an electric furnace and by blowing oxygen upon the molten steel under atmospheric pressure. Also the process enables the preparation of extremely low carbon stainless steel which has hitherto been difficult to be prepared commercially by conventional processes due to the oxidation loss of Cr resulted from the markedly high refining temperature.
The Witten process has, however, the following disadvantages:
1. Since the decarburization is carried out in a closed vessel under reduced pressure, its rate is so high that the sampling of molten steel is not only difficult during the course of the reaction but also techniques for the rapid analysis of the molten steel have not been established which can be completed during the process of decarburization.
2. In order to avoid any explosion which may occur due to water leakage, the Witten process employs a consuming type lance which is steel pipe as such or coated with refractory mortar in place of a water cooled lance in a closed vessel under reduced pressure so that the consumption thereof cannot be observed directly but the lance is advanced empirically. Such advance of lance does not always correspond to the actual consumption thereby causing the decarburization rate to fluctuate severely with the change in the height of the lance above the molten steel surface so that the final C value can not be estimated correctly from the vacuum achieved, and,
3. There has been provided a method for estimating the C-value in the molten steel every moment from the carbon amount exhausted from the system by measuring continuously the contents of CO and CO.sub.2 in the exhaust gas by gas analyses and the flow rate of the exhaust gas. Since such a method requires calibration due to the gas density since not only the exhaust gas contains moisture but also the composition fluctuates in the course of decarburization to cause substantial error in the calibration. Further, some technical difficulties in the closed vessel used in commercial applications under reduced pressure are encountered in the measurement of flow rate rather than the gas analyses of such an exhaust gas.
There has arisen some problems in vacuum refining with oxygen lancing such that the correct C-value cannot be obtained during the refining process to correctly control it and also the control of the decarburization is difficult. The control of decarburization which has been carried out most generally during the actual operation is to calculate the oxygen amount required for the oxidation of C, Si and the like from the analysis of the molten steel composition prior to the oxygen lancing under reduced pressure, estimating the required oxygen amount with reference to the oxygen efficiency obtained empirically as shown, for example, in FIG. 1, and to control the decarburization from the estimated oxygen demand. In such a method, since the decarburization efficiency of the oxygen depends largely on the temperature, the Si-content prior to the oxygen lancing, the amount of slag, the flow rate of oxygen, the degree of vacuum, the degree of agitation by argon gas and the like, the decarburization has been carried out by calibrating the temperature and silica content empirically, treating the slag so as to provide a uniform slag thickness by removing provisionally, for example, in the ladle and on the basis of the calculated amount of lancing oxygen during the stepwise change in the flow rate of oxygen, degree of vacuum and agitation by means of argon gas in order to minimize the dispersion in the decarburization rate during the heating.
Even though the above-mentioned conditions are intended to be maintained constantly, when the oxygen lance is consumed at a larger speed than that fed into the furnace as mentioned above and oxygen gas is blown onto the molten metal surface from a relatively larger height, it is exhausted without colliding against the molten steel surface. Hence there may occur a case where the exhaust gas includes unburned oxygen gas in a large amount. In such a case, the oxygen efficiency will be decreased to induce some dispersion in the relation between the final C-value and the degree of achieved vacuum due to the variation of unburned oxygen gas during the heating to cause inevitably an error in the estimation of the final C-value based on the degree of vacuum. The amount of unburned oxygen becomes significant when the C-value is reduced to a value of, for example, less than 0.10% during the last stage of decarburization and the amounts of formed CO and CO.sub.2 are decreased correspondingly thereto to give a large deviation in the degree of achieved vacuum.
Consequently, the decarburization is carried out to achieve a final C-value lower than the desired value and then carburizing the product to the desired C-value. By using such measures, not only expensive alloy elements such as Cr, Mn, etc. are lost by the oxidation during carburization but also various disadvantages are associated therewithin. For example, the procedure increases the oxygen content of the molten steel thereby deteriorating its quality, promotes loss of refractoriness due to unnecessary temperature increase, prolongs the lancing time and reduces the efficiency of the process.